The invention relates to methods and apparatus for tuning resonant cavities, and particularly to systems in which reflected power and/or standing wave ratios are sensed by circuitry, especially microprocessor controlled circuitry, to effectuate generating of motor control signals that adjust an internal tuning element in the cavity to minimize power reflected from the cavity.
Transmitter combiners are multiple tuned cavity devices which allow simultaneous transmission of signals from a plurality of transmitters at different but closely spaced frequencies by means of a single antenna. Transmitter combiners include one tuned cavity for each frequency, typically. Each of the tuned cavities is coupled by a coaxial cable to a separate transmitter and is also coupled to a common coaxial connector to which a single antenna cable is connected leading to the antenna. Recently, the mobile telephone communication technology has been rapidly growing. Numerous holders of FCC licenses have established "cells" or regions that are typically several miles in extent, each having low power transmitters generally centered in a particular cell. A major problem that has always existed for transmitter combiners, and for tuned cavities in general, is the fact that numerous external influences cause the tuned cavities to become detuned, substantially increasing their insertion losses and decreasing the amount of transmitter power that reaches the transmitter antenna. Variations in critical dimensions of tuned cavities due to normal changes in the temperature of the cavities has always been a major problem. To avoid detuning of cavities due to temperature, tuned cavities, especially square prism filters, commonly have been made of Invar, which is an expensive metal that must be coated with copper to provide the high surface conductivity required in a tuned cavity that is useful in the high frequency bands assigned for mobile communications. Variations in humidity in many parts of the country are great enough to severely detune the cavities of transmitter combiners, and variations in atmospheric pressure also can significantly detune such cavities. Up to now, there has been no practical means of compensating for such detuning, and users of mobile communications equipment have had to live with the fact that often an inadequate amount of the transmitter power actually reaches the transmitting antenna. Mobile communications systems commonly utilize one of the transmitter channels as a "signaling channel" which controls transmission of messages between the cell site and the mobile units to effectuate switching operations to determine on which channel a particular cell and mobile unit shall operate. If the signaling channel becomes inoperative for any reason, the entire cell becomes inoperative until repairs can be made. Although it is possible to reassign channel frequencies remotely, allowing one of the voice channels to operate as a signaling channel, up to now it has been impossible to remotely make corresponding changes in the resonant frequencies of the various cavities of a transmitter combiner. Therefore, whenever it has been advantageous to reassign channel frequencies, it has been necessary for a technician to travel to the antenna site and manually tune the cavities to the newly assigned transmitter frequencies. This is an expensive and time consuming procedure which it would be very desirable to eliminate.
It is clear that there is a unmet need for an economical, automatic, self-tuning tuned cavity which continuously and automatically self-tunes its resonant frequency to the frequency of the incoming transmitter signal, regardless of the influence of external factors such as temperature, humidity, atmospheric pressure, etc.